![]() Except for form tolerances like Straightness, Flatness, Circularity, and Cylindricity, all other geometrical tolerancing can refer to a datum to explain the design intent. ![]() Datum feature C must contact the feature control frame third.Datum plays an important role in GD & T. Datum feature B must contact the feature control frame second. So per the Feature Control Frame below, datum feature A must contact the feature control frame first. The Datum Feature Simulators must be contacted in the order stated in our Feature Control Frame. Now that we have our datum features established, we are ready to specify in which order to put them in contact with the Datum Feature Simulators. The physical objects that the Datum Features contact when measuring are Datum Feature Simulators. Datums are perfect planes (in this section). So the things on the physical part are datum features. But the Y14.5 standard says that they are called Datum Feature Simulators. Personally, I think they should be called Datum Simulators because they are actually simulating the datums. The physical objects we use to simulate datums are called Datum Feature Simulators. We must simulate the planes with gage fixtures or other physical things. Of course, we cannot measure to perfect, theoretical planes. In this section our datums are planes - perfect, theoretical planes. Datums are perfect, theoretical entities. ![]() The measurement will be repeatable.Īlso note that datum features and datums are two different things. Now the part is constrained in all six degrees of freedom. This will constrain the "X" degree of freedom. Third we push the part against the tertiary datum plane. This will constrain the "Z" degree of freedom and also the "v". Second we push the part against the secondary datum plane. This will constrain the "Y" degree of freedom and also the "u" and the "w". So the steps are:įirst set the part on the primary datum plane. Because of imperfections in the part, we may get different answers if we were to push the part against the tertiary datum plane before the secondary datum plane instead of pushing it against the secondary datum plane before pushing it against the tertiary datum plane.Īt this point we will also consider degrees of freedom being constrained. In order to ensure that the measurement is repeatable, we not only specify that the part be set on and against the three datum planes, we also specify in which order to set the part against the datum planes. The three mutually perpendicular planes are called a Datum Reference Frame. Note that we are measuring from the planes, we are not measuring directly from the sides of the block. Our strategy is to set the block on and/or against three mutually perpendicular planes. We want to measure where the hole is, and we want the measurement to be repeatable. When this is done, there must be a note specifying exactly how the clamping is to be done so that the deformation will be exactly the same each time the part is measured.Ĭonsider the part on the right side below. This is frequently done with sheet metal parts or other non rigid parts. But since the sheet metal is non rigid, we may be able to force all four feet to rest on the plane by clamping them in place. If we were to make a sheet metal statue of the dog, once again the four feet would never rest in the same plane at the same time. Likewise when measuring a part, if we set the part on a perfect plane, it will rest on the three high points. So the dog statue at any one time would rest on three of the four feet. But if we were to make a rigid statue of the dog and set it on a perfectly flat plane, we would never get all four feet to be in exactly the same plane. All four feet touch the ground because the dog adjusts how he stands. The ground acts as a datum.Ĭonsider some additional issues. The dog stands on the ground, and we measure up from the ground. If we want to measure the height of a dog, we don't measure directly from the bottom of the dog's feet.
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